During eye examinations it is necessary to precisely position the patient's eye relative to corrective optics employed in the examining process. Historically, a patient's eyes were examined by positioning interchangeable lenses in front of the patient's eye to determine what, if any, sight correction the patient requires. To obtain accurate examination results it is, of course, necessary to precisely position the corrective lenses relative to the patient's eye. This was done with mechanical indicators that were positioned on the side of the patient's head. The examiner had to visually align the indicator with the outer surface of the patient's cornea to place the corrective optics at the desired distance from the patient's eye. This process was relatively inaccurate and most cumbersome because it required the positioning of mechanical measuring devices on the side of the patient's head and further because it required the examiner to observe the patient from his side. The task was both time-consuming and subject to relatively large error rates because the reading was influenced and therefore changed by changes in the position of the examiner relative to the patient's eye and the measuring device. Nevertheless, such measuring devices were widely used in the past because they could be directly attached to the lens holders positioned in front of the patient's face.
U.S. Pat. No. 3,874,774 discloses and claims a new eye testing apparatus which is a radical departure from previously used devices. One of the important features of the testing apparatus disclosed in that patent is the elimination of all lens holders and other measuring devices in front of the patient's eye. Instead, an examination light beam is directed through corrective optics having variable spherical and astigmatic inputs. The corrective optics are placed at a location remote from the patient. Focusing optics in the form of a field mirror are placed between the corrective optics and the patient to focus to the patient a real image of the corrective optics.
When performing an eye examination with this testing apparatus it is of course of the utmost importance that the patient's eyes are at the proper location relative to the focal point of the field mirror to avoid inaccurate or false readings which would render the examination worthless. However, the testing apparatus does not have lens holders in front of the patient's face to which a mechanical measuring device could be attached; consequently it would require additional structure to incorporate prior art eye positioning devices of the type discussed above. More importantly, if such devices were used they would defeat one of the primary objects of the testing apparatus, namely, to provide an unobstructed view of the patient's face, and to remove obstructions from the patient's field of view.